An epithelial-derived factor inhibits contraction in canine perfused bronchial segments

Contractility and Ca2+ signaling of smooth muscle cells in different generations of mouse airways

The control and mechanisms of airway smooth muscle cell (SMC) contraction were investigated with a sequential series of lung slices from different generations of the same airway from the cardiac lobe of the mouse lung. Airway contraction was measured by monitoring the changes in airway lumen area with phase-contrast microscopy. Changes in intracellular calcium concentration of the SMCs were studied with a custom-built confocal or two-photon microscope. The distribution of the airway SMCs and the muscarinic M(3) or 5-HT(2A) receptors was determined with immunofluorescence. Methacholine and 5-HT induced a concentration-dependent airway contraction and Ca(2+) oscillations within the SMCs of each airway generation. The airway contraction in response to the same agonist concentration was greater in the middle generation compared with the distal or proximal generations of the same airway. Similarly, the Ca(2+) oscillations varied in different generations of the same airway, with a slower frequency in the SMCs of the distal zone as compared with the middle or proximal zones of airways. By contrast, high KCl induced minimal contraction and very slow Ca(2+) oscillations throughout the whole intrapulmonary airway. The slower agonist-induced Ca(2+) oscillations in the distal zone correlated with a reduced expression of agonist receptors. The layer of SMCs increased in thickness in the middle and proximal zones. These results indicate that the contractility of airway SMCs varies at different positions along the same airway and that this response partially results from different Ca(2+) signaling and the total amount of the SMCs.

Airway basement membrane perimeter in human airways is not a constant; potential implications for airway remodeling in asthma

Many studies that demonstrate an increase in airway smooth muscle in asthmatic patients rely on the assumption that bronchial internal perimeter (P(i)) or basement membrane perimeter (P(bm)) is a constant, i.e., not affected by fixation pressure or the degree of smooth muscle shortening. Because it is the basement membrane that has been purported to be the indistensible structure, this study examines the assumption that P(bm) is not affected by fixation pressure. P(bm) was determined for the same human airway segment (n = 12) fixed at distending pressures of 0 cmH(2)O and 21 cmH(2)O in the absence of smooth muscle tone. P(bm) for the segment fixed at 0 cmH(2)O was determined morphometrically, and the P(bm) for the same segment, had the segment been fixed at 21 cmH(2)O, was predicted from knowing the luminal volume and length of the airway when distended to 21 cmH(2)O (organ bath-derived P(i)). To ensure an accurate transformation of the organ bath-derived P(i) value to a morphometry-derived P(bm) value, had the segment been fixed at 21 cmH(2)O, the relationship between organ bath-derived P(i) and morphometry-derived P(bm) was determined for five different bronchial segments distended to 21 cmH(2)O and fixed at 21 cmH(2)O (r(2) = 0.99, P < 0.0001). Mean P(bm) for bronchial segments fixed at 0 cmH(2)O was 9.4 +/- 0.4 mm, whereas mean predicted P(bm), had the segments been fixed at 21 cmH(2)O, was 14.1 +/- 0.5 mm (P < 0.0001). This indicates that P(bm) is not a constant when isolated airway segments without smooth muscle tone are fixed distended to 21 cmH(2)O. The implication of these results is that the increase in smooth muscle mass in asthma may have been overestimated in some previous studies. Therefore, further studies are required to examine the potential artifact using whole lungs with and without abolition of airway smooth muscle tone and/or inflation.

Factors that determine acetylcholine responsiveness of guinea pig tracheal tubes

Acetylcholine administered to the inside of epithelium-denuded tracheal tubes did cause a potent contraction (2486+/-120 mg). In contrast, a response was hardly observed in tissues with an intact epithelial layer (674+/-81 mg), which was due to both the synthesis of nitric oxide and the activity of acetylcholinesterase, since the contractions to acetylcholine were significantly enhanced after preincubation with N(omega)-nitro-L-arginine methyl ester (L-NAME) or physostigmine (1374+/-65 and 1120+/-65 mg, respectively). In addition, the suppressive effect was caused by the barrier function of the epithelial layer, since preincubation of epithelium-denuded tissues with physostigmine significantly increased the pD2 value for acetylcholine (7.48+/-0.04) compared to intact tissues preincubated with physostigmine (6.32+/-0.10) and epithelium-denuded preparations without physostigmine (6.37+/-0.06). Increasing concentrations of physostigmine administered to the inside of tissues with epithelium did induce a potent spontaneous contraction (1440+/-350 mg) that was prevented by atropine. In contrast to what was expected, the contractile response was diminished in tracheal tubes without epithelium (665+/-221 mg). It is concluded that contractions of epithelium-denuded tissues are more pronounced to exogenous than to endogenous acetylcholine, and that the production and breakdown of this neurotransmitter is very rapid in intact guinea pig airways. Moreover, the release of nitric oxide and the barrier function of the epithelium did suppress the responsiveness to acetylcholine.